A DAC is a device that will output an analogue parameter, usually a voltage, current, or charge, that is representation of a digital input to it. Thus an 8 bit digital word can have one of 256 values, so an 8 bit DAC will output a voltage or current which can have one of 256 different values. Since the analogue values which can be output are limited in number, any particular analogue value produced is accurate only within upper and lower limits. Thus, digitally, each one of the 256 different values is precisely defined, whereas the analogue output can only have a defined degree of accuracy associated with it. There is not absolute agreement on the precise definition of accuracy, but an 8 bit; 8 bit accuracy DAC will output analogue voltage or currents which are +/-0.5 lsbs accurate.
Multibit high accuracy oversampling ACDs require DACs which have relatively few bits (say 4 to 8) and are very accurate (say +/-0.01 lsbs). Another way of describing the accuracy required is that a DAC of n bits with m bit accuracy is needed, where m&gt;n. Such DACs are very difficult to construct and 16 bits for m is the currently available limit, although in theory 18 bits or more would be required.
There exist many ways of constructing DACs. For example, DACs may incorporate current sources, where the current sources have different values. One conventional technique is, for an 8 bit DAC, to use 8 current sources which are each a factor of 2 larger than the next one. One digital input control line is then connected to each current source. Another technique, feasible with the advent of VLSI, for the same 8 bit DAC, is to use 255 equal current sources, where the msb digital control line switches on 128 sources, the next switches 64, the next 32, and so on, as diagrammatically indicated in FIG. 1. It is to be noted that an n bit DAC needs at least 2.sup.n -1 current sources. Various combinations of these approaches have also been used, such as 16 equal current sources, followed by another 4 sources which are each half the magnitude of the preceding one, for an 8 bit DAC.
Oversampling ADCs put a low accuracy ADC with an accurate DAC in a feedback loop, and sample the signal many (e.g. 128) times faster than the required rate. The effect of the feedback loop is to correlate up quantising noise from the ADC in such a way that the noise spectrum is shaped, and is very low in the required part of the spectrum. A digital filter on the output of the ADC then filters out all signals (including noise) from the unwanted parts of the spectrum, and then decimates the output data rate, frequency shifting it if necessary. The wanted part of the spectrum is usually, but not always, the low frequency part (e.g. the first 1/128th) of the spectrum.